Some time in the spring of 1858, Teresa Sickles began an affair with Key.[2] Dan Sickles, though a serial adulterer himself, had accused his much-younger wife of adultery several times during their five-year marriage, but she had repeatedly denied it to his satisfaction.[4] But then Sickles received a poison pen letter[9] informing him of his wife's affair with Key.[10][2][4] He confronted his wife, who confessed to the affair.[2] Sickles then made his wife write out her confession on paper.[11] Sickles saw Key sitting on a bench outside the Sickles home on February 27, 1859, signalling to Teresa, and confronted him.[12][2][4][11] Sickles rushed outside into Lafayette Square, cried "Key, you scoundrel, you have dishonored my home; you must die",[13] and with a pistol repeatedly shot the unarmed Key.[2][4] Key was taken into the nearby Benjamin Ogle Tayloe House, where he died some time later.[14]

Sickles was acquitted on the basis of temporary insanity, a crime of passion, in one of the most controversial trials of the 19th century.[15] It was the first successful use of the defense in the United States.[16] Sickles' attorney, Edwin Stanton, later became the Secretary of War. Newspapers declared Sickles a hero for "saving" women from Key.[16] Years later, while attending the theater in New York City, Sickles became aware of the presence of Key's son James Key in the audience; both men watched each other throughout the performance. Nothing happened.[17]